Align the source and sink behaviours using the `@attributes` annotation

[pierre-borckmans]

Description:
Currently, the @attributes annotation cannot be used with sinks the same way they can be used with sources.

For instance, this mapping

@map(type='json', enclosing.element="$ @attributes(symbol = "company.symbol", price = "price", volume = "volume"))

would correctly parse the following json, when used on a source:

{
    "company" : {
        "symbol": "AAPL"
    },
    "price": 1234,
    "volume": 1000
}

but does not work when used one a sink, to produce the same json from for instance the following stream:

define stream TestStream(symbol string, price int, volume int)

[suhothayan]

@pierre-borckmans currently the option we have for sinks is to use @payload

@sink(type='...',  
        @map(type='json', enclosing.element='$',
                    @payload( """
{
    "company" : {
        "symbol": "{{symbol}}"
    },
    "price": {{price}},
    "volume": {{volume}}
}""")))
define stream BarStream (symbol string, price float, volume long);

We'll try to incorporate your suggestion in future releases by constructing the JSON from XPath.